Various methods have been proposed for preparation of sweepstakes contest awards, lottery tickets, promotional game cards, premium cards, and the like, containing hidden entries, such as numerals, messages, symbols, or the like, which can be revealed to the recipient by various means.
For example, U.S. Pat. No. 4,726,608 to Walton discloses the use of an opaque coating over hidden indicia. The image of the indicia is later made visible by scratching off the opaque coating or by applying a solvent to disperse the coating. This system has the disadvantages of either requiring extraneous solvents or producing unwanted dust.
Co-pending U.S. application Ser. No. 07/987,710 to John C. H. Chang entitled "Heat Sensitive System and Use Thereof" filed on even date herewith, the disclosure of which is hereby incorporated herein by reference, discloses a heat sensitive chromogenic system in the production of documents to prevent unauthorized or fraudulent use of a xerographic color copier for reproduction of negotiable instruments, such as checks, money orders and the like. When, for example, a check bearing the heat sensitive chromogenic coating in a verification thereof is presented for payment, the bank teller can simply apply heat to the verification area, and if a visible image symbol appears in the verification area, the teller will know that the check is an original.